Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $6.45   9.575%   $7.0676   $7.07   $7.05   $7.10  
  Purchase #2 $170.77   5.235%   $179.7098   $179.71   $179.70   $179.70  
  Purchase #3 $209.36   13.482%   $237.5859   $237.59   $237.60   $237.60  
  Purchase #4 $220.59   13.384%   $250.1138   $250.11   $250.10   $250.10  
  Purchase #5 $156.92   7.463%   $168.6309   $168.63   $168.65   $168.60  
  Purchase #6 $131.85   6.868%   $140.9055   $140.91   $140.90   $140.90  
  Purchase #7 $6.97   6.452%   $7.4197   $7.42   $7.40   $7.40  
  7 purchase totals = $902.91       $991.4332   $991.44   $991.40   $991.40